Measurements of volatile and residue burning times of captive coal particles have shown them to vary in proportion to the square of the initial particle diameter; this was true for each of 10 coals tested. For the volatiles, the measured constant of proportionality was found to be independent of coal rank when allowance was made for swelling, the burning rate apparently being controlled by the diffusional rate of volatile escape through a porous matrix of fixed carbon. For the residues, the experimental “square-law” relation confirms Nusselt’s prediction; and agreement between predicted and experimental values of the proportionality constant was found to be good. An apparent rank dependence, showing shorter burning times with less fixed carbon, was simply a consequence of there being less fixed carbon to burn. There was no influence of rank-dependent “reactivity.” The relevance of the results to flames is also discussed.

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